Dehydrogenation, dehydrocyclization, and dehydrocyclodimerization processes involve the removal of hydrogen from hydrocarbons to form alkenes and/or aromatics. The olefin and aromatics yields and throughputs are constrained by thermodynamic limitations, including endothermic and H2 formation. Specifically, they are limited by the amount of heat that can be introduced without causing significant thermal cracking, and by how low the operating pressures can be without incurring excess compression cost (capital and utility).
One solution to the problem of hydrogen formation is to oxidize it. For example, U.S. Pat. No. 4,788,371 describes a dehydrogenation process which includes selective oxidation of the hydrogen produced which is used to generate heat to increase the temperature to the level needed for the next dehydrogenation reaction section. A single catalyst was used for both dehydrogenation and oxidation. The reaction could be performed in a reactor with multiple beds or in different reactors for the dehydrogenation and oxidation reactions. The need to add oxygen or air handling equipment increased the cost of the process. In addition, the oxidation produced water which had to be removed.
Another solution was the use of catalytic membrane reactors which combine chemical reactions with membrane separation. The membranes can be polymeric, metallic, and inorganic. However, the use of catalytic membrane reactors makes the process more complex, and more difficult to model and optimize due to the combined reaction and separation processes. Polymeric Membranes in Catalytic Reactors, Chem. Rev. 2002, 102, 3779-3810; Thin Hydrogen-Selective SAPO-34 Zeolite Membranes for Enhanced Conversion and Selectivity in Propane Dehydrogenation Membrane Reactors, Chem. Mater. 2016, 28, 4397-4402.
U.S. Pat. No. 9,776,935 describes a process involving dehydrogenation and membrane separation. The dehydrogenation reaction mixture is subjected to membrane separation in a separate unit. The use of separate units for dehydrogenation and membrane separation is said to allow independent control of the process conditions in each unit. In order to avoid problems with the stability of the membranes, the membrane separation unit is maintained slightly below 500° C., preferably in the range of 420-490° C., and more preferably 450-470° C. The dehydrogenation reaction takes place at temperatures of 450-750° C., preferably 500-750° C., and most preferably 550-750° C. to attain techno-economically viable conversions per pass to minimize the capital and operating cost to purify the product and recycle the unconverted hydrocarbon, respectively. Operating the membrane separation unit at less than 500° C. requires additional interstage heating to increase the retentate to the dehydrogenation conditions of the next reactor. It could also require cooling the reaction mixture to obtain the desired separation temperature. These steps increase the overall cost of the process and incur additional thermal cracking.
Therefore, there is a need for a process for making olefins, aromatics, and alkenylaromatics in which the temperature of the reaction mixture is maintained at reaction conditions during the separation process.